Ignition magneto safety interlock

ABSTRACT

Circuits for controlling the starting of an internal combustion engine that has a magneto ignition system including a primary winding that can be shorted to prevent ignition. The circuits include a position-responsive switch and an electronically controlled switch to selectively control a shorting switch that is, in use, connected across the primary winding of a magneto. In the preferred arrangement, the shorting switch is normally nonconductive and is maintained nonconductive (1) when the position-responsive switch is closed, indicative of a condition in which it is acceptable to start the engine, or (2) when the electronically controlled switch is changed from its normal level of conductivity in response to the running of the engine. When the position-responsive switch is opened before the motor is started, the normal condition of the electronically controlled switch permits the shorting switch to be made conductive by the spark producing pulse and thereby prevents ignition. The position-responsive switch is actuated through a transmission of the engine being controlled and in the embodiment shown is normally closed when the transmission is in neutral.

mite States Patent [191 Howard IGNITION MAGNETO SAFETY Inventor:

Assignee:

Filed:

Appl. No.:

[52] 11.5. C1. ..123/179 K, 123/179 13G, 123/118,

123/148 S, 123/102, 123/198 DC, 74/843,

[ 51 Apr. 10, 1973 Primary ExaminerLaurence M. Goodridge Assistant Examiner-Rona1d B. Cox AttorneyWatts, Hoffman, Fisher & Heinke [5 7] ABSTRACT Circuits for controlling the starting of an internal combustion engine that has a magneto ignition system including a primary winding that can be shorted to prevent ignition. The circuits include a positionresponsive switch and an electronically controlled switch to selectively control a shorting switch that is,

74/850 in use, connected across the primary winding of a 51 Int. Cl ..F02n 17/00, F02b 77/00, F02p 9/00 magnem In the preferred arrangement, the shorting Fleld 0f S, i h i nomlally nonconductive and s ta ed 1 3/179 179 74/843 850 nonconductive (1) when the position-responsive 56 R f ed switch is closed, indicative of a condition in which it is 1 e erences It acceptable to start the engine, or (2) when the elec- UNITED STATES PATENTS tronically controlled switch is changed from its normal level of conductivity in response to the running of the 3,601,103 8/1971 Sw1den g w h positiorbresponsive Switch i Opened 352L612 7/1970 23/ before the motor is started, the normal condition of 2,934,054 4/1960 Quinlan ..l23/179 n n h th h 2,695,366 11/1954 Coffey ..123/179 F eeQmmCa Y comm ed Pemlts e S 3,168,891 2/1965 Cook ..123/179 mg swltch to be made conductlve by the Spark 2,807,729 9/1957 Redick ..l23/179 producing pulse and thereby prevents ignition. The

3,581,720 6/1971 Hemphill ..l23/179 position-responsive switch is actuated through a trans- 3563-219 2/ Miel'as -123/179 mission of the engine being controlled and in the em- 3,356,082 12/1967 Jukes 123/179 bodiment shown is normally closed when the transmis sion is in neutral.

27 Claims, 4 Drawing Figures T T w I i I I I [2/ 24 I l I 1 /CI 56R 1 I J! 62) fij I 3 x I t MPC I R? I J I I 2 I I6-I i E 2/ 22 P5 Re I T3] I Dz l l j I I; I T4 IGNITION MAGNETO SAFETY INTERLOCK BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to circuits for controlling the operation of internal combustion engine ignition systems that utilize magnetos.

2. Prior Art The present state of the art of safety interlocks for magneto ignition engines is believed to be exemplified by U. S. Pat. No. 3,521,612. As disclosed therein, it is known to ground a magneto primary of an engine in response to the closing of a manually operable switch so that the engine should not start when a transmission connected to the engine is in gear. Provision is made for preventing the grounding of the magneto primary winding in response to the closing of the switch after the engine is running by utilizing a positive, nonsparkproducing, portion of the primary winding ignition pulse to energize a relay and open a switch in series with the manually operable switch in the grounding circuit.

As recognized in the aforementioned patent, small engines of the type used on lawn mowers, garden tractors, and similar equipment typically include a manually operable control to couple the engine to a wheel drive of the machine, and a dangerous situation exists if the engine is started when the engine is coupled to the drive. As is also recognized, where a safety interlock is provided to prevent engine starting when the machine is in an unsafe condition for starting, the interlock must be dependable against mechanical failure and operator interference, because the operator will tend to rely upon the interlock. The safety mechanism or interlock must therefore withstanding operating conditions to which the machine is subjected, without failure, such as extremes of heat and cold, vibrations,

and the like, with minimum maintenance. It must also be low in cost and must not require engine modification or adversely affect engine performance.

While the above desirable features have been recognized, circuits and components provided to achieve such features, for example, as disclosed in the aforementioned patent, fail to a significant extent to achieve such desiderata.

Among the disadvantages of known interlock systems are the following: (1) the use of moving parts, such as relay contacts, in circuit components, which are subject to mechanical failure due to a variety of causes, (2) the use of relay coils for operating moving parts, such as control switches, (3) the use of pulses other than and of opposite polarity from the spark producing pulse to operate control switches, thus requiring such an additional pulse from the magneto which may or may not be available depending upon the engine manufacturer and which may have a particular function that is adversely affected by the additional function, as when it is used in connection with a low impedance relay circuit, and (4) the use of a switch that must be open to permit starting, as when the transmission is in neutral, and which is closed to prevent starting, and hence is not fail-safe in the event a mechanical malfunction prevents the switch from closing, and which is not compatible with the use of additional switches in a safety loop, responsive to additional conditions, any one of which is to render the engine incapable of starting.

The need of a relay coil to integrate circuit pulse signals in order to operate is especially disadvantageous in connection with its use in a safety interlock system in which the relay is energized by the integration of pulses generated after the engine has been started to open a switch that permits shifting the associated transmission into drive without causing the safety switch to short the ignition. For the circuit to provide the desired safety feature, the relay must not be energized in response to 0 the starting RPM if the transmission is in gear. Yet, if

the relay is to stay energized while the engine idles at relatively low RPM, it may well be actuated by the pulses produced by the starting RPM and by-pass the safety switch. While this can be overcome by requiring a high idling speed and a relay that is energized only by such higher pulse rate, such a relay will tend to de-energize and short the ignition when the engine RPM is low, as when the engine is under heavy load.

SUMMARY OF THE INVENTION The present invention overcomes the above-mentioned and other disadvantages of the prior art through an electrical safety interlock system that eliminates moving parts and coils, that utilizes the voltage pulse that generates the spark in the ignition system rather than other pulses to operate certain circuit components, that has a high impedance so as to minimize the effect of the system on the voltage pulse utilized, that uses a position-responsive switch that, in the preferred arrangement, must be in a closed position in order to start the engine and is therefore essentially fail-safe as compared with a normally opened switch, and that permits a safety loop having other switches that must be closed to permit starting of the engine. Further, the circuits and circuit components comprising the system are small in size and immune to vibration, extremes of temperature, and the like so that they can be located on the engine or transmission with which they are directly associated, rather than on other parts of the related machine, which would require separate installation.

The above advantages and features are accomplished with a first circuit that is capable of grounding or shorting the primary winding of a magneto ignition system through a first electronic control means, such as an SCR, connected across the winding and that can be selectively changed from a nonconductive to a conductive state through a control electrode. This change is effected by a voltage pulse and is controlled by a second circuit having a position-responsive switch and a second electronic control means, such as a field-effect transistor, and a third circuit having a voltage sensitive memory, such as a capacitor. Only the pulse that produces the ignition spark is used by the circuits.

The position-responsive switch in one position isolates the control electrode of the first electronic control means from the voltage pulse, so that the first control means remains nonconductive. As a result, a voltage pulse produced in the primary winding will induce a sparking pulse in the secondary winding of the magneto and tire the engine. The position of the positionresponsive switch in which it isolates the control electrode of the first control means from the voltage pulse is preferably closed, for reasons that will be explained in more detail subsequently, and the closed condition exists when the transmission is in neutral, a safe condition for the machine when the engine is started.

In another or second position of the position-responsive switch, the switch acts in combination with the second electronic control means to prevent ignition of the engine by permitting a pulse from the magneto primary winding to reach the control electrode of the first electronic control means in the first or shorting circuit, so that the first control means conducts and shorts the primary magneto winding. The position-responsive switch is prevented from shorting the ignition once the engine is running and the transmission is shifted into gear (which would place the switch in its second position), by the voltage sensitive memory of the third circuit. This memory applies a voltage signal to the control electrode of the second electronic control means to change it from its normal condition of conductivity if voltage pulses from the magneto were initiated when the position-responsive switch was in said first position and maintains the changed condition as long as the voltage pulse rate from the magneto primary is of a predetermined frequency, just great enough to be indicative of a running condition of the engine but not so low as to permit restarting the engine within the interval between pulses of the predetermined frequency. Within such range, there is no danger that the memory will cause the circuit to revert to the nonrunning condition when the engine idles or lugs under extreme load. When the voltage pulse rate is lower than such minimum frequency (for example, when the engine is turned off), the memory no longer applies a signal to the second electronic control means, which then reverts to its original condition so that shorting of the primary winding is again responsive to the position of the position-responsive switch.

Particular advantages accrue from the circuits of the present invention and the circuit components utilized therein. These advantages include (I) an absence of moving parts, except for the position-responsive switch, (2) the system can be and preferably is constructed so that the position-responsive switch must be closed in order to permit starting of the engine and, as a result, the condition is readily and reliably changed mechanically, as when the transmission is shifted into gear, with less risk of a malfunction than if a switch must be closed to activate the safety feature, and further will readily permit a safety loop of additional switches in series, any one of which, if opened, will prevent starting, (3) the circuit is voltage sensitive rather than frequency sensitive within all operating ranges of the engine so that it can be utilized at low engine RPM without danger of shorting out the engine, and is initially operable from a single voltage pulse, (4) the entire system provides a high electrical impedance and can therefore function off of the pulse used to fire the spark plug, permits use of the circuits with magnetos that do not generate a pulse of opposite polarity from the spark generating pulse, provides a minimum drain on the magneto-produced energy, will not affect the positive portion of the pulse normally required for degaussing, and will not have an adverse affect on the so-called ringing of the spark pulse, which finds a principal beneficial use in starting cold engines, and (5) the circuit is of extremely small size, not adversely effected by vibrations and extremes in temperatures, is highly reliable, and has a longer life expectancy than circuits utilizing relay coils and relay operated switches.

From the above, it will be apparent that an object of this invention is to provide a system for preventing the starting of internal combustion engines that have magneto ignition systems under predetermined conditions, manifested by the condition of a position-responsive switch and related circuits, which is fail-safe, reliable, has a long life expectancy, is capable of operating off of the spark-generating pulse from a magneto ignition system, has little or no effect on magneto pulses of opposite plurality from that utilized to generate the spark pulse, and can be embodied in a compact, economical, unit capable of being attached directly to the engine or engine transmission.

This and other objects, features and advantages of this invention will become more apparent from the detailed description that follows, when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of an engine, transmission and safety interlock system embodying the present invention;

FIG. 2 is a side elevational view of a plug-in housing containing circuit components of the interlock system of the present invention and which connects to a safety switch and magneto of a transmission and engine in the manner illustrated in FIG. 1;

FIG. 3 is a block diagram of the electrical circuits comprising the interlock system of the present invention; and

FIG. 4 is a schematic wiring diagram of the safety interlock system of the present invention.

DETAILED DESCRIPTION With reference to FIG. 1, an internal combustion engine l0 and an associated drive transmission 12 are shown, exemplary of a variety of engine driven equipment, such as garden tractors, lawn mowers, etc., that utilize small, usually one-cylinder, internal combustion engines with magneto ignitions. Typical of such engines, the engine 10 includes a shut-off switch 14 which, when closed, will ground the primary coil of a magneto ignition to turn off the engine. The transmission 12 is a type that is manually shiftable between a neutral condition and a drive condition.

For safety of operation, the engine 10 should not be started unless the transmission is in neutral, so that the equipment driven by the engine and transmission will not begin moving or cutting or otherwise operative while the engine is being started. To assure that the engine is not started while the transmission is in a drive position, a safety interlock system is provided that includes a two-position transmission-operated switch 16 associated with the manually operated shifting mechanism of the transmission 12, and control circuits housed in a small unit 18 and connected to the magneto ignition system of the engine 10 and to the switch 16. The condition of the switch 16 is responsive to the position of the transmission shifting mechanism and the control circuits and switch are connected to allow starting of the engine only when the transmission is in neutral. In the preferred embodiment, the transmission switch is normally closed when the transmission is in a neutral condition.

The unit 18 comprising circuits and circuit components of the safety interlock system is supported either on the engine driven device, on the engine itself or on the transmission. The circuits and components comprising the safety interlock system will be described in more detail subsequently. In the general arrangement shown in FIG. 1, the unit 18 is electrically connected to one side of the primary coil of the magneto of the engine by a conductor 20, to the transmission switch 16 by conductors 21 and 22, and is grounded or connected to the other side of the primary magneto coil by a conductor 23. It should be understood that the conductors 20-23 terminate in a receptacle having the necessary electrical connections and the unit 18 is provided with snap-in terminals received in the receptacle for electrically connecting the circuits and circuit components of the unit 18 to the conductors and circuit components directly associated with the engine and transmission. Optional additional normally closed switches 24, 25, 26 are shown in series with the transmission switch 16 in the circuit formed by the conductors 21 and 22, to provide a logic system safety loop. Such switches are provided when it is desired that certain other conditions be met before the engine 10 can be started. These other conditions may include, for example, that the operator be seated, and/or that a guard be lowered over a cutting mechanism, or other specific feature applicable to the particular device driven by the engine and transmisson.

A preferred construction of the unit 18, as shown in FIG. 2, includes a cup-like aluminum housing 25 in which circuit components of the interlock system are potted with a suitable potting compound. Three electrical snap-on terminals 26, 27, 28 extend from the housing, along with the wire 23. In a typical embodiment, the cup or housing 25 is approximately 1 inch in diameter and slightly less than 1 inch in height. Because the circuit components are all solid state and securely housed within the cup 25, with a potting compound for example, the circuit (1) is capable of functioning within temperature limits between minus 40F. and plus 200 F., (2) has a life expectancy of at least l00,000 hours, and (3) is not affected by engine or transmission vibration.

Three principle functional aspects of the safety interlock system of the present invention are illustrated by the block diagram of FIG. 3. As illustrated, a solid state switch circuit illustrated by block 34 is connected across tbe magneto primary coil MPC. Depending upon the engine and magneto circuit, which may vary among different manufacturers, one side of the magneto and one side of the switch circuit may be connected to ground potential. Operation of the switch circuit selectively shorts out (i.e., provides a low impedance across) the primary magneto coil and prevents ignition by preventing a voltage pulse from being induced in the secondary coil. of the magneto, which produces the ignition spark in the engine. The solid state switch circuit is operated by a condition-responsive control circuit illustrated by block 36 and a voltage-sensitive memory circuit. illustated by block 38. The condition-responsive control circuit 36 is responsive to both the condition of the engine 10 (i.e., running or not running) and the position of the transmission switch 16, and controls the condition of the solid state switch of circuit 34 in direct response to the input conditions. One of the input conditions is established by the voltage sensitive memory circuit 38, which is sensitive to the running of the engine and prevents the switch circuit 34 from shorting the magneto coil when the transmission is shifted into a drive condition after the engine is started.

Specific circuits forming the safety interlock system for preventing starting of the engine 10 when the transmission switch 16 is open and for permitting the engine to run, once started, notwithstanding subsequent opening of the switch 16, are shown in detail in FIG. 4. As diagrammatically illustrated, portions of the circuits comprise the unit 18, and those portions are shown within a broken line rectangle. Those circuit portions are connected with circuit components associated more directly with the engine and/or transmission, which are shown outside the broken line rectangle, through interconnections at terminals T1, T2, T3 and T4, which correspond respectively to thesnap on terminals 26, 27, 28 and the ground wire 23 of the unit 18.

A magneto primary coil MPC is connected between the terminals T1 and T4. This generalized form may be more specifically achieved with one side of the coil MPC and the terminal T4 separately connected to ground potential. The position sensitive switch 16 actuated by the transmission 12 is connected between the terminals T2 and T3.

With reference to the unit 18, a resistor R1 and a capacitor C1 are series connected between the terminal T1 and a juncture J1. The anode of a diode D1 is connected to the juncture J1 and the cathode is connected to the terminal T3. The anode of a second diode D2 is connected to the terminal T3 and the cathode is connected to the terminal T4.

A resistor R2 is connected between the juncture J1 and a juncture J2. A resistor R3 is connected from the juncture J2 to the anode of the diode D2.

The anode of a zener diode is connected to the juncture J2, between the resistors R2 and R3. The cathode of the zener diode Z1 is connected to the anode of the diode D2.

A resistor R4 is connected between the terminal T1 and the terminal T2. A cathode of a zener diode Z2 is connected to the terminal T2 and the anode is connected to the anode of the diode D2.

A field-effect transistor Q1 is connected through a source terminal to the terminal T2 and through a drain terminal to a juncture J3. The gate of the field-effect transistor Q1 is connected to a juncture J4. The juncture 14 is common to the anode of the zener diode Z1 and the juncture J2.

A capacitor C2 and a resistor R5 are series connected between the juncture J4 and the anode of the diode D2.

A resistor R6 is connected between the juncture J3 and the anode of the diode D2.

A silicon controlled rectifier SCRl is connected through its anode to the terminal T1, through its cathode to the anode of the diode D2, and through its gate to the juncture J3.

Suitable values for the components referred to above for use with commercial magneto type engines, such as the type used on riding lawn mowers and the like, are indicated in the table below:

TABLE COMPONENT VALUE Resistor R1 lOK Ohms Resistor R2 22 Megohms Resistor R3 10 Megohms Resistor R4 4.7K Ohms Resistor R5 1K Ohms Condenser C1 0.02 Microfarads Condenser C2 100 Picofarads Zener diode Z1 12 volts Zener diode Z2 6 volts In operation, the circuits described above (1) serve to short the magneto primary coil if the transmission 12 is in gear when the engine 10 is started, (2) prevent shorting of the magneto primary coil as long as the transmission remains in neutral, and (3) permit the transmission to be shifted out of neutral once the engine is started without shorting the magneto primary coil. In connection with the latter function, the circuits allow the engine to run at the lowest RPM of which it is capable without prematurely reacting as if the engine had stopped, thereby shorting the ignition and causing the engine to stall under load. At the same time, the circuits revert to a condition in which the magneto primary coil is shorted so that the engine cannot be started with the transmission in gear, within a time interval too short to permit the restarting of a stalled engine while the transmission is in gear.

Assuming the transmission 12 is in gear so that the switch 16 is open, and the engine is not running, the field-effect transistor Q1 will be in its normally conductive condition and will remain so until a voltage is applied to the gate. The silicon controlled rectifier SCRl is in its normally nonconductive condition and will remain so until a voltage is applied to its gate. The engine is mechanically cranked and an initial pulse is produced by the primary winding MPC of the magneto. All positive pulses from the magneto are blocked by the diode D2. The negative voltage pulse, which is used to induce the spark pulse in the secondary winding of the magneto, is applied to the terminal T4, resulting in a potential at T1 which is positive with respect to the terminal T4, With the switch 16 open, the pulse produces a rise in potential at the juncture J3 of the gate to the SCRl, via the resistor R4 and the field-effect transistor Q1. This causes the silicon controlled rectifier SCRl to conduct. This occurs early during the generation of the pulse, because only a low potential is required to switch SCRl. Consequently, SCRl conducts before the potential between T1 and T4 is of substantial magnitude. The low potential between T1 and T4 plus the high impedance of the resistor R1 prevents the capacitor C1 from being charged sufficiently to affect the field-effect transistor Q1. (The manner in which capacitor C1 affects the transistor Q1 will be explained in more detail subsequently.) The conduction of the pulse through SCRl eliminates the potential at the terminal T1 and prevents the primary coil from inducing a sparkgenerating pulse in the secondary coil of the magneto. Because SCRl is rendered conductive at the low potential, a very low voltage is established across the resistors R2 and R3, resulting in a low voltage at the juncture J2. Accordingly, the field-effect transistor Q1 sees a voltage under no-start conditions that is too small to cause it to change from its conductive to a nonconductive condition.

If the transmission is in neutral at the time the engine is started, the switch 16 is closed, the field-effect transistor Q1 is conductive, and the silicon controlled rectifier SCRl is nonconductive. By requiring that the switch 16 be closed in order to start the engine, a malfunction of the switch tends to result in a fail-safe condition in that a malfunction is more likely to prevent switch contacts from closing than to prevent them from opening. (E.g., a primary cause of malfunction may be due to foreign matter between contacts, corrosion, etc.) An initial pulse is produced by the coil MPC in the manner previously described, but now a circuit is completed from the terminal Tl through the resistor R4 and the switch 16 to the terminal T3 and thence through the diode D2 and the terminal T4, back to the coil MPC. As a result, no signal is produced at the juncture J3 or the gate of the silicon controlled rectifier SCRl, so that SCRl remains nonconductive. In addition, the resistor R4 provides sufficient impedance to essentially maintain the potential between the terminals T1 and T4. Consequently, the potential at T1 increases to a maximum condition and charges the capacitor C1 via the resistor R1. Moreover, the pulse,

not being shorted through the silicon controlled rectifier, SCRl, produces a corresponding pulse in the secondary winding of the magneto so that the engine is fired.

Charging of the capacitor C1 is facilitated by the diode D1, which also serves to limit the discharge path to the high impedance circuit components. Thus, the capacitor C1 establishes a current through the resistors R2 and R3, which act as a voltage divider to establish a desired potential at the juncture J2, which is connected to the juncture J4 and to the zener diode Z1. The zener diode Z1 limits the potential at the juncture J4 and the gate of the field-effect transistor O1 to 12 volts. The voltage at the juncture J4 is also protected against trantients or stray currents by the filter circuit comprised of the capacitor C2 and the resistor R5. As a result of the potential established at the junctures J2 and J4 and applied to the gate of the field-effect transistor Q1, the transistor is rendered nonconductive. Consequently, when the transmission is shifted into gear, so as to open the switch 16, the voltage pulses (i.e., the potential between terminals T1 and T4) will not produce a signal at the juncture J3 via the resistor R4 and field-effect transistor Q1, and will not switch the silicon controlled rectifier SCRl to a conductive condition and the primary magneto coil is not shorted.

The zener diode Z2 limits the voltage that can be applied between the resistor R4 and tbe field-effect transistor Q1 to 6 volts. The combination of the two zener diodes Z1 and Z2 limits the total potential difference between the source and gate to no more than about 18 volts, well within the rating of the transistor. Accordingly, the zener diodes function as protective devices for the field-effect transistor Q1.

The circuit that applies a signal to the gate of the field-effect transistor Q1 functions as a memory circuit. The capacitor C1 is charged by each spark generating voltage pulse produced by the coil MPC and maintains the charge at a sufficient level for the time interval that exists between successive spark generating pulses while the engine is running, to maintain the transistor O1 in a nonconductive state. Preferably, the capacitor will maintain such a charge for a period of not less than 0.2

second and no more than 2 seconds. Thus, between pulses well within the frequency at which they are produced when the engine is running, the capacitor remembers that the engine is running so as not to allow the transistor Q1 to revert to its normal conductive state. If the time interval between pulses is sufficient to allow the condenser C1 to lose its charge, the transistor Q1 reverts to its previous condition of conductivity and the next pulse from the magneto ignition coil will provide a signal at the juncture J3, causing SCRl to conduct and short the ignition. This result is desired, because the magnitude of the time delay indicated that the engine had stopped running and restarting must be prevented as long as the switch 16 remains open, indicating that the transmission is still in gear.

With a capacitor C1 that will hold a charge for only the minimum time interval acceptable, i.e., 0.2 second, the engine can nevertheless run at its minimum RPM without causing the circuit to revert to a condition where the next ignition pulse will cause the magneto coil to be shorted. With a capacitor that will hold a charge for the maximum time interval, i.e., 2 seconds, there is little sacrifice in safety because within that period of time the engine could not practically be restarted if it stalled. Because the typical internal combustion engine will not run in response to ignition pulses at two second intervals and typically not at intervals of 0.2 second, the circuitry described above in connection with the present safety interlock system is not frequency sensitive within the running range of the engine.

Within the scope of the present invention, circuit modifications are contemplated and have been constructed for achieving substantially the same results in a substantially similar manner. For example, a normally nonconductive NPN transistor could be substituted for the field-effect transistor Q1 in the gate control circuit of the silicon controlled rectifier SCRl. By connecting such a transistor in parallel with the switch 16, it would function as a holding circuit for the switch when rendered conductive by ignition pulses. Thus, upon opening the switch 16 after the engine is started, the transistor would provide an alternate equivalent circuit to prevent an ignition pulse from switching the SCRl to a conductive state. Accordingly, with the switch 16 closed, the SCRl would not be switched as in the embodiment already described. With the switch 16 open, but with the engine not running, the NPN transistor in parallel with the switch would be nonconductive, would not by-pass the switch 16, and the pulse would trigger SCRl. Once the engine is running, the transistor would be rendered conductive and would by-pass the switch 16, which could then be opened when the transmission is shifted from neutral into a drive condition, without causing the silicon controlled rectifier from being triggered.

In the event a normally open position-sensitive switch is used in place of the switch 16, the switch is connected between the resistor R4 and the source electrode of the field-effect transistor Q] as by connecting the source of the transistor Q1 to the terminal T3 rather than directly to the resistor R4.

Typically, the voltage pulses produced by the magneto will have a large positive peak followed by a large negative peak, followed by an oscillating condition of reduced and decreasing magnitude, referred to as ringing." In certain magneto designs, an additional pulse will be provided both before and after the peak pulses and ringing pulses described, of opposite sense or polarity from the spark producing pulse, for specific purposes, such as degaussing. The circuits of the present system, which are limited in their affect only to the pulse that causes the spark plug of the engine to fire, in no way reduce the effectiveness of the other pulses of opposite polarity for their intended purposes.

Moreover, by not requiring these additional pulses for operation, the present system is usable with all magnetos.

While the present invention has been described with oarticularity, it should be understood that various modifications and alterations may be made therein without departing from the spirit and scope of the invention set forth in the appended claims.

What is claimed is:

1. A safety interlock for controlling the starting of an internal combustion engine that has a magneto ignition system with a primary winding that can be shorted to prevent ignition; said interlock comprising: first electrical circuit means for connection to a primary winding of an ignition system, including a first electronic control means having a first, second and control electrode, operative to short said winding in response to a selectively applied electrical signal to said control electrode; second control circuit means including a positionresponsive switch and also including a second electronic control means coupled between the control and second electrodes of said first electronic control means, for selectively permitting a signal to be applied or preventing a signal from being applied to the control electrode of said first electronic control means in response to the condition of the position-responsive switch or the second electronic control means or both, said second electronic control means having a first, second and control electrode and being changeable between conditions exhibiting high and low electrical impedances; and third electrical circuit means for applying an electrical signal to the control electrode of said second electronic control means to determine its condition; the electrical operation of all of said circuit means being dependent solely upon magnetogenerated energy.

2. A safety interlock as set forth in claim 1 wherein said position-responsive switch is movable to a closed position to prevent a signal from being applied to the control electrode of said first electronic control means and wherein said third circuit means applies an electrical signal to change the condition of said second electronic control means only when said position-responsive switch is closed.

3. A safety interlock as set forth in claim 2 wherein said second electronic control means is in series electrically with the control electrode of said first electronic control means.

4. A safety interlock as set forth in claim 3 wherein the electrical impedance of said second electronic control means is normally low in the absence of a signal to its control electrode.

5. A safety interlock as set forth in claim 2 wherein said third circuit means includes capacitance means responsive to periodic electrical imput pulses for applying an electrical signal to the control electrode of said second electronic control means for sufficient time to maintain a changed condition of said second electronic control means for the interval between said periodic pulses while said engine is running.

6. A safety interlock as set forth in claim 1 wherein said third electrical circuit means includes a capacitor for applying a voltage from the primary magneto winding to said control electrode of the second electronic control means for a time interval of between 0.2 and 2 seconds.

7. An electrical system for selectively shorting a primary winding ofa magneto ignition system for an internal combustion system, comprising: first circuits means including a first electronic control means for connection across said winding and controllable through a control electrode between a low impedance or conductive state and a high impedance or non-conductive state in response to a voltage signal; second circuit means for controlling the application of a voltage signal produced in said primary winding to said control electrode, including a position-responsive switch and a second electronic control means for selectively permitting a voltage signal to be applied to said control electrode; and third circuit means, including a voltage sensitive memory, for applying a control signal to said second electronic control means in response to voltage signals produced in said primary winding while said first electronic control means is maintained nonconductive, to change the conductivity of said second electronic control means from that which it exhibits in the absence of a control signal, and maintaining the changed conductivity only as long as a voltage pulse rate above a predetermined level is produced in said primary winding, thereby controlling the condition of said first electronic control means independently of said position-responsive switch when said voltage pulse rate is at said predetermined level; the electrical operation of all of said circuit means being dependent solely upon magneto-generated energy.

8. An electrical circuit for selectively shorting only negative electrical pulses produced in a primary winding of a magneto in an ignition system of an internal combustion engine, comprising: a first electrical circuit connected to a primary winding of a magneto in an ignition system and including first electronic control means having a first, second and control electrode and operative when an electrical signal is applied to the control electrode to conduct in only one direction; a second electrical circuit connected to said winding and coupled to said control electrode for selectively controlling the conductivity of said first electronic control means; rectifier means for permitting only negative voltage pulses from said winding to be applied to said circuits; said second circuit including a positionresponsive switch operable to selectively prevent or allow said first electronic control means from being or to be rendered conductive and a second electronic control means operable to selectively prevent or allow said first electronic control means from being or to be rendered conductive when said position-responsive switch is in a position to allow the same, both of said switch and second electronic control means being coupled between the control and second electrodes of said first electronic control means, said second electronic control means having a control electrode and being operable between a condition exhibiting high electrical impedance and a condition exhibiting low electrical impedance in response to an electronic signal applied to its control electrode; and a third circuit means for applying an electrical signal to the control electrode of said second electronic control means, initially only when said position-responsive switch is operated to prevent said first electronic control means from being rendered conductive, to change the condition of said second electronic control means to prevent said first electronic control means from being rendered conductive notwithstanding the subsequent condition of said position responsive switch.

9. A safety interlock for controlling the starting of an internal combustion engine that has a magneto ignition system with a primary winding that can be shorted to prevent ignition, said interlock comprising: first electrical circuit means for connection to a primary winding of an ignition system, including a first electronic control means having a first, second and control electrode, operative to short said winding in response to a selectively applied electrical signal to said control electrode; second electrical control circuit means including a position-responsive switch and also including a second electronic control means coupled between the control and second electrodes of said first electronic control means, for selectively permitting a signal to be applied to the control electrode of said first electronic control means in response to one condition of the positionresponsive switch and one condition of the second electronic control means, said second electronic control means having a first, second and control electrode and conditions exhibiting high and low electrical impedances, dependent upon an electrical signal at the control electrode; and third electrical circuit means for applying a signal to the control electrode of said second electronic control means to determine its condition.

10. A magneto safety interlock for controlling the starting of an internal combustion engine that has a magneto primary winding that can be shorted to prevent ignition, said interlock comprising: means, including a circuit adapted to be connected electrically to a primary winding of a magneto, to selectively short the winding, said circuit including a switch electrically operable between a condition exhibiting high electrical impedance and a condition exhibiting low electrical impedance and when connected across said winding necessarily shorts the winding when the switch is in a low impedance condition; and an electrical control circuit coupled to said switch and adapted to be connected electrically to said winding for selectively changing said switch from a condition exhibiting a high electrical impedance to a condition exhibiting a low electrical impedance, including connections for electrically coupling a position-responsive switch that, when closed, prevents the condition of said electricallyoperable switch from being so changed; and electrical circuit means coupled to said electrically operable switch to prevent the condition of said electrically operable switch from being changed in response to opening said position-responsive switch when electrical pulses are being generated by said winding; the electrical operation of said interlock being dependent solely upon magneto-generated energy.

11. A magneto safety interlock for controlling the starting of an internal combustion engine that has a magneto primary winding that can be shorted to prevent ignition, said interlock comprising: means, including a circuit adapted to be connected electrically to a primary winding of a magneto, to selectively short the winding, said circuit including a switch electrically operable between a condition exhibiting high electrical impedance and a condition exhibiting low electrical impedance and when connected across said winding necessarily shorts the winding when the switch is in a low impedance condition; and an electrical control circuit, including an electronic control device, coupled to said switch and adapted to be connected electrically to said winding for selectively changing said switch from a condition exhibiting a high electrical impedance to a condition exhibiting a low electrical impedance, and further including connections for electrically coupling a position-responsive switch that, when closed, prevents the condition of said electrically operable switch from being so changed; and electrical circuit means coupled to said electrically operable switch for affecting the condition of the electronic control device to prevent the condition of said electrically operable switch from being changed in response to opening said position-responsive switch when electrical pulses are being generated by said winding; the electrical operation of said interlock being dependent solely upon magneto-generated energy.

12. A high impedance electrical circuit for use with a magneto ignition system of an internal combustion engine and an associated drive transmission to selectively short a primary winding of the magneto ignition system to prevent starting of the engine in response to a predetermined condition of the drive transmission associated with the engine, the condition and operation of said circuit being in part responsive to spark producing voltage pulses produced by said magneto ignition system; said circuit, when electrically coupled across a primary magneto winding, being electrically associated therwith in the following manner, dependent in operation solely upon magneto-generated energy and including: coupling means for electrically connecting said circuit to a primary winding of a magneto ignition system and to a mechanically actuatable switch; a silicon controlled rectifier havingrfirst, second and control electrodes, being normally non-conductive in the absence of a voltage signal applied to the control electrode, and connected to said coupling means for being electrically connected across the primary winding of a magneto ignition system; a transistor having first, second and control electrodes, adapted to be switched by a voltage applied to its control electrode between conductive and non-conductive conditions, one of which is its normal condition'prior to operation of energization of the circuit, electrically connected to the control electrode of the silicon controlled rectifier and electrically connected to said coupling means for connection to said primary winding and said switch, and

operable in its conductiveconditionto electrically couthe control electrode of the transistor of a magnitude and duration to maintain a switched condition of said transistor from its normal condition prior to operation or energization of said circuit, as long as the frequency of said spark-producing voltage pulses is above a predetermined level, not less than one every two seconds, thereby controlling the condition of said silicon controlled rectifier independently of an electrical connection across said coupling means for connecting said circuit to a mechanically actuatable switch.

14. In combination, an internal combustion engine with a magneto ignition system that has a primary winding that can be shorted to prevent ignition, a manually shiftable drive transmission connected to said engine, a position responsive safety switch connected to and operated by said transmission, and an ignition interlock for controlling the starting of the internal combustion engine, said interlock being dependent in operation solely upon magneto-generated energy and comprising: first circuit means including a first electronic control means connected across said winding and controllable through a control electrode between a low impedance or conductive state and a hightimpedance or nonconductive state in response to a voltage signal; second circuit means for controlling the application of a voltage signal produced in said primary winding to said control electrode, including said position-responsive switch and a second electronic control means for selectively permitting a voltage signal to be applied to said control electrode; and third circuit means, including a voltage sensitive memory, for applying a signal to said second electronic control means and affecting the conductivity thereof as long as a voltage pulse rate'above a predetermined level is produced in said primary winding, thereby controlling the condition of said first electronic control means independently of said position-responsive switch when said voltage pulse rate is at said predetermined level.

15. A magneto safety interlock for controlling the starting of an internal combustion engine that has a magneto primary winding that can be shorted to prevent ignition, said interlock comprising: means, including a circuit adapted to be connected electrically to a primary winding of a magneto, to selectively short the winding, said circuit including an electronic switch electrically operable through a control electrode between a condition exhibiting high electrical impedance and a condition exhibiting low electrical impedance; and a control circuit coupled to said switch and adapted to be connected electrically to said winding for selectively changing said switch from a condition exhibiting a high electrical impedance to a condition exhibiting a low electrical impedance, including a position-responsive switch that, when closed, prevents the condition of said electrically-operable switch from being so changed; and electrical circuit means coupled to said electrically operable switch to prevent the condition of said electrically operable switch from being changed in response to opening said position-responsive switch when electrical pulses are being generated by said winding, said circuit means including a capacitor and serving to selectively isolate the control electrode of said electrically operable switch from positive pulses from said magneto primary winding; the electrical operation of said circuits and said electrical circuit means being dependent generated energy.

16. A safety interlock for controlling the starting of an internal combustion engine that has a magneto ignition system with a primary winding that can be shorted to prevent ignition; said interlock comprising: a first electronic control means electrically connected across a primary winding of an ignition system, normally nonconductive and rendered conductive by an electrical potential applied to a control electrode thereof; a mechanically actuated switch and a second electronic control means, both electrically coupled to the primary winding and to the first electronic control means to control the electrical potential at said control electrode, said switch having a first condition that prevents an electrical potential from being applied to said control electrode and a second condition that does not prevent an electrical potential from being applied to said control electrode, and said second electronic control means having a condition in which it does not prevent an electrical potential from being applied to said control electrode, and a condition responsive to running of the engine in which it prevents an electrical potential from being applied to said control electrode; and circuit means electrically connected to the primary winding and the second electronic control means, responsive to electrical pulses generated through said ignition system by the running of the engine, to apply an electrical potential to a control electrode of said second electronic control means to establish the condition of said second electronic control means in which it prevents an electrical input to the control electrode of said first electronic control means, whereby a change from said first condition of said switch subsequent to the starting of said engine will not render said first electronic control means conductive.

17. A safety interlock as set forth in claim 16 wherein said second electronic control means is in series electrically with the control electrode of said first electronic control means.

18. A safety interlock as set forth in claim 17 wherein the electrical impedance of said second electronic control means is normally low in the absence of a signal to its control electrode.

19. A safety interlock for controlling the starting of an internal combustion engine that has a magneto ignition system with a primary winding that can be shorted to prevent ignition; said interlock comprising: A first electronic control means electrically connected across a primary winding of an ignition system, normally nonconductive and rendered conductive by an electrical potential applied to a control electrode thereof; a mechanically actuated switch and a second electronic control means, both electrically coupled to the primary winding and to the first electronic control means to control the electrical potential at said control electrode, said switch having a closed condition that prevents an electrical potential from being applied to said control electrode and an open condition that does not prevent an electrical potential from being applied to said control electrode, and said second electronic control means having a condition in which it does not prevent an electrical potential from being applied to said control electrode, and a condition responsive to running of the engine in which it prevents an electrical solely upon magnetopotential from being applied to said control electrode; and circuit means electrically connected to the primary winding and the second electronic control means, responsive to electrical pulses generated through said ignition system by the running of the engine, to apply an electrical potential to a control electrode of said second electronic control means to establish the condition of said second electronic control means in which it prevents an electrical input to the control electrode of said first electronic control means, whereby an opening of said switch subsequent to the starting of said engine will not render said first electronic control means conductive.

20. A safety interlock as set forth in claim 19 wherein said circuit means includes capacitance means responsive to periodic electrical input pulses for applying an electrical signal to the control electrode of said second electronic control means for sufficient time to maintain a changed condition of said second electronic control means for the interval between said periodic pulses while said engine is running.

21. A safety interlock for controlling the starting of an internal combustion engine that has a magneto ignition system with a primary winding that can be shorted to prevent ignition; said interlock comprising: a first electronic control means electrically connected across a primary winding of an ignition system, normally nonconductive and rendered conductive by an electrical potential applied to a control electrode thereof; a mechanically actuated switch and a second electronic control means, both electrically coupled to the primary winding and to the first electronic control means to control the electrical potential at said control electrode, said switch having a first condition that prevents an electrical potential from being applied to said control electrode and a second condition that does not prevent an electrical potential from being applied to said control electrode, and said second electronic control means having a condition in which it does not prevent an electrical potential from being applied to said control electrode, and a condition responsive to running of the engine in which it prevents an electrical potential from being applied to said control electrode; and circuit means electrically connected to the second electronic control means, responsive to the running of the engine, to apply an electrical potential to a control electrode of said second electronic control means to establish the condition of said second electronic control means in which it prevents an electrical input to the control electrode of said first electronic control means, whereby a change from said first condition of said switch subsequent to the starting of said engine will not render said first electronic control means conductive.

22. A safety interlock for controlling the starting of an internal combustion engine that has a magneto ignition system with a primary winding that can be shorted to prevent ignition; said interlock comprising: a circuit to be electrically connected across a primary winding of an ignition system and connected to contacts of a position responsive switch that in a first condition permits a potential to be maintained across the primary winding so that the engine can be started and in a second condition prior to the starting of the engine prevents such a potential from being maintained; said circuit including: a first electronic control means, with first, second and control electrodes, that is nonconductive until a gating signal is applied to the control electrode, and that when connected across the primary winding of an ignition system and when conductive, will short the winding to prevent the ignition from firing; a second electronic control means with first, second and control electrodes, and a conductive and nonconductive condition dependent upon the presence or absence of an electrical signal at the control electrode, which signal originates from said primary winding, said second electronic control means in one said condition permitting a gating signal to be effectively applied to the control electrode of said first electronic control means, said gating signal originating from the primary winding when the position responsive switch is in said second condition, and said second electronic control means preventing a gating signal from being effectively applied to the control electrode of said first electronic control means when said second electronic control means is in the other of said conditions, regardless of the condition of said position responsive switch; and means, coupled between said primary winding and the control electrode of said second electronic control means, responsive to electrical pulses from said primary winding, to apply an electrical signal to said last-mentioned control electrode to change the condition of said second electronic control means after the engine has started and to maintain a signal as long as said electrical pulses occur at a predetermined minimum frequency, thereby preventing a gating signal from being applied at the control electrode of said first electronic control means when said position responsive switch is changed to said second condition.

23. A magneto safety interlock for an engine ignition system, including a silicon controlled rectifier for connection across a primary magneto winding of an engine ignition system, said silicon controlled rectifier being normally nonconductive and which when conductive serves to electrically short said winding; first electrical circuit means coupled to a control electrode or gate of said silicon controlled rectifier and for connection to a position responsive switch and also across said primary winding for changing the conductive condition of said silicon controlled rectifier in response to the conductive condition of said switch; and second electrical circuit means coupled to said control electrode or gate,- responsive to running of said engine, for preventing the conductive condition of said silicon controlled rectifier from being changed in response to the conductive condition of said switch after said engine is running.

24. A magneto safety interlock for selectively shorting electrical pulses only of a polarity used for ignition and produced in a primary winding of a magneto in an ignition system of an internal combustion engine, comprising: a silicon controlled rectifier for connection across a primary magneto winding of an engine ignition system, said silicon controlled rectifier being normally nonconductive and having a gate electrode that renders said silicon controlled rectifier conductive in response to an electrical signal; and electrical circuit means coupled to said silicon controlled rectifier, including (a) rectifier means for permitting the passage through said circuit means of only electrical pulses of the polarity used for firing said engine, (b) means for coupling said gate electrode to said winding through a positionresponsive switch that permits or prevents the change of the conductive condition of said silicon controlled rectifier in response to an electrical pulse prior to the running of the engine, depending upon the condition of said switch, and (c) capacitor means responsive to electrical pulses from said primary winding after the engine is running for preventing said silicon controlled rectifier from being rendered conductive by a condition of said position responsive switch.

25. A safety interlock for controlling the starting of an internal combustion engine that has a magneto ignition system with a primary winding that can be shorted to prevent ignition; said interlock comprising: an electronic control means electrically connected across a primary winding of an ignition system, normally nonconductive and rendered conductive by an electrical potential applied to a control electrode thereof; a control circuit including a mechanically actuated switch electrically coupled to the primary winding and to the first electronic control means to control the electrical potential at said control electrode, said switch having a closed condition that prevents an electrical potential from being applied to said control electrode and an open condition that does not prevent an electrical potential from being applied to said control electrode; and circuit means responsive to running of the engine to prevent an electrical input from being applied to the control electrode of said electronic control means, whereby an opening of said switch subsequent to the starting of said engine will not render said electronic control means conductive.

26. A safety interlock as set forth in claim 25 wherein said circuit means includes a capacitor coupled between said primary winding and said control electrode, charged by pulses generated through said primary winding, for preventing an electrical input to the control electrode of said electronic control means when said engine is running.

27. An electrical circuit for selectively shorting only negative electrical pulses produced in a primary winding of a magneto in an ignition system of an internal combustion engine, comprising: a first electrical circuit connected to a primary winding of a magneto in an ignition system and including first electronic control means having a first, second and control electrode and operative when an electrical signal is applied to the control electrode to conduct in only one direction; a second electrical circuit connected to said winding and coupled to said control electrode for selectively controlling the conductivity of said first electronic control means; rectifier means for permitting only negative voltage pulses from said winding to be applied to said circuits; said second circuit including a positionresponsive switch operable to selectively prevent or allow said first electronic control means from being or to be rendered conductive and a second electronic control means operable to selectively prevent or allow said first electronic control means from being or to be rendered conductive when said position-responsive switch is in a position to allow the first electronic control means to be rendered conductive, said second electronic control means having a control electrode and being operable between a condition exhibiting high electrical impedance and a condition exhibiting low electrical impedance in response to an electronic signal condition it exhibits in the absence of a signal applied to its control electrode to prevent said first electronic control means from being rendered conductive notwithstanding the subsequent condition of said position responsive switch. 

1. A safety interlock for controlling the starting of an internal combustion engine that has a magneto ignition system with a primary winding that can be shorted to prevent ignition; said interlock comprising: first electrical circuit means for connection to a primary winding of an ignition system, including a first electronic control means having a first, second and control electrode, operative to short said winding in response to a selectively applied electrical signal to said control electrode; second control circuit means including a positionresponsive switch and also including a second electronic control means coupled between the control and second electrodes of said first electronic control means, for selectively permitting a signal to be applied or preventing a signal from being applied to the control electrode of said first electronic control means in response to the condition of the position-responsive switch or the second electronic control means or both, said second electronic control means having a first, second and control electrode and being changeable between conditions exhibiting high and low electrical impedances; and third electrical circuit means for applying an electrical signal to the control electrode of said second electronic control means to determine its condition; the electrical operation of all of said circuit means being dependent solely upon magneto-generated energy.
 2. A safety interlock as set forth in claim 1 wherein said position-responsive switch is movable to a closed position to prevent a signal from being applied to the control electrode of said first electronic control means and wherein said third circuit means applies an electrical signal to change the condition of said second electronic control means only when said position-responsive switch is closed.
 3. A safety interlock as set forth in claim 2 wherein said second electronic control means is in series electrically with the control electrode of said first electronic control means.
 4. A safety interlock as set forth in claim 3 wherein the electrical impedance of said second electronic control means is normally low in the absence of a signal to its control electrode.
 5. A safety interlock as set forth in claim 2 wherein said third circuit means includes capacitance means responsive to periodic electrical imput pulses for applying an electrIcal signal to the control electrode of said second electronic control means for sufficient time to maintain a changed condition of said second electronic control means for the interval between said periodic pulses while said engine is running.
 6. A safety interlock as set forth in claim 1 wherein said third electrical circuit means includes a capacitor for applying a voltage from the primary magneto winding to said control electrode of the second electronic control means for a time interval of between 0.2 and 2 seconds.
 7. An electrical system for selectively shorting a primary winding of a magneto ignition system for an internal combustion system, comprising: first circuits means including a first electronic control means for connection across said winding and controllable through a control electrode between a low impedance or conductive state and a high impedance or non-conductive state in response to a voltage signal; second circuit means for controlling the application of a voltage signal produced in said primary winding to said control electrode, including a position-responsive switch and a second electronic control means for selectively permitting a voltage signal to be applied to said control electrode; and third circuit means, including a voltage sensitive memory, for applying a control signal to said second electronic control means in response to voltage signals produced in said primary winding while said first electronic control means is maintained nonconductive, to change the conductivity of said second electronic control means from that which it exhibits in the absence of a control signal, and maintaining the changed conductivity only as long as a voltage pulse rate above a predetermined level is produced in said primary winding, thereby controlling the condition of said first electronic control means independently of said position-responsive switch when said voltage pulse rate is at said predetermined level; the electrical operation of all of said circuit means being dependent solely upon magneto-generated energy.
 8. An electrical circuit for selectively shorting only negative electrical pulses produced in a primary winding of a magneto in an ignition system of an internal combustion engine, comprising: a first electrical circuit connected to a primary winding of a magneto in an ignition system and including first electronic control means having a first, second and control electrode and operative when an electrical signal is applied to the control electrode to conduct in only one direction; a second electrical circuit connected to said winding and coupled to said control electrode for selectively controlling the conductivity of said first electronic control means; rectifier means for permitting only negative voltage pulses from said winding to be applied to said circuits; said second circuit including a position-responsive switch operable to selectively prevent or allow said first electronic control means from being or to be rendered conductive and a second electronic control means operable to selectively prevent or allow said first electronic control means from being or to be rendered conductive when said position-responsive switch is in a position to allow the same, both of said switch and second electronic control means being coupled between the control and second electrodes of said first electronic control means, said second electronic control means having a control electrode and being operable between a condition exhibiting high electrical impedance and a condition exhibiting low electrical impedance in response to an electronic signal applied to its control electrode; and a third circuit means for applying an electrical signal to the control electrode of said second electronic control means, initially only when said position-responsive switch is operated to prevent said first electronic control means from being rendered conductive, to change the condition of said second electronic control means to prevent said first electronic control means from being rendered conductive notwithstanding the subsequent condition of said position responsive switch.
 9. A safety interlock for controlling the starting of an internal combustion engine that has a magneto ignition system with a primary winding that can be shorted to prevent ignition, said interlock comprising: first electrical circuit means for connection to a primary winding of an ignition system, including a first electronic control means having a first, second and control electrode, operative to short said winding in response to a selectively applied electrical signal to said control electrode; second electrical control circuit means including a position-responsive switch and also including a second electronic control means coupled between the control and second electrodes of said first electronic control means, for selectively permitting a signal to be applied to the control electrode of said first electronic control means in response to one condition of the position-responsive switch and one condition of the second electronic control means, said second electronic control means having a first, second and control electrode and conditions exhibiting high and low electrical impedances, dependent upon an electrical signal at the control electrode; and third electrical circuit means for applying a signal to the control electrode of said second electronic control means to determine its condition.
 10. A magneto safety interlock for controlling the starting of an internal combustion engine that has a magneto primary winding that can be shorted to prevent ignition, said interlock comprising: means, including a circuit adapted to be connected electrically to a primary winding of a magneto, to selectively short the winding, said circuit including a switch electrically operable between a condition exhibiting high electrical impedance and a condition exhibiting low electrical impedance and when connected across said winding necessarily shorts the winding when the switch is in a low impedance condition; and an electrical control circuit coupled to said switch and adapted to be connected electrically to said winding for selectively changing said switch from a condition exhibiting a high electrical impedance to a condition exhibiting a low electrical impedance, including connections for electrically coupling a position-responsive switch that, when closed, prevents the condition of said electrically-operable switch from being so changed; and electrical circuit means coupled to said electrically operable switch to prevent the condition of said electrically operable switch from being changed in response to opening said position-responsive switch when electrical pulses are being generated by said winding; the electrical operation of said interlock being dependent solely upon magneto-generated energy.
 11. A magneto safety interlock for controlling the starting of an internal combustion engine that has a magneto primary winding that can be shorted to prevent ignition, said interlock comprising: means, including a circuit adapted to be connected electrically to a primary winding of a magneto, to selectively short the winding, said circuit including a switch electrically operable between a condition exhibiting high electrical impedance and a condition exhibiting low electrical impedance and when connected across said winding necessarily shorts the winding when the switch is in a low impedance condition; and an electrical control circuit, including an electronic control device, coupled to said switch and adapted to be connected electrically to said winding for selectively changing said switch from a condition exhibiting a high electrical impedance to a condition exhibiting a low electrical impedance, and further including connections for electrically coupling a position-responsive switch that, when closed, prevents the condition of said electrically operable switch from being so changed; and electrical circuit means coupled to said electrically operable switch for affecting the condition of the electronic control device to prevent the condition of said electrically operable switch from being changed in response to opening said position-responsive switch when electrical pulses are being generated by said winding; the electrical operation of said interlock being dependent solely upon magneto-generated energy.
 12. A high impedance electrical circuit for use with a magneto ignition system of an internal combustion engine and an associated drive transmission to selectively short a primary winding of the magneto ignition system to prevent starting of the engine in response to a predetermined condition of the drive transmission associated with the engine, the condition and operation of said circuit being in part responsive to spark producing voltage pulses produced by said magneto ignition system; said circuit, when electrically coupled across a primary magneto winding, being electrically associated therwith in the following manner, dependent in operation solely upon magneto-generated energy, and including: coupling means for electrically connecting said circuit to a primary winding of a magneto ignition system and to a mechanically actuatable switch; a silicon controlled rectifier having first, second and control electrodes, being normally non-conductive in the absence of a voltage signal applied to the control electrode, and connected to said coupling means for being electrically connected across the primary winding of a magneto ignition system; a transistor having first, second and control electrodes, adapted to be switched by a voltage applied to its control electrode between conductive and non-conductive conditions, one of which is its normal condition prior to operation of energization of the circuit, electrically connected to the control electrode of the silicon controlled rectifier and electrically connected to said coupling means for connection to said primary winding and said switch, and operable in its conductive condition to electrically couple said control electrode of the silicon controlled rectifier to said coupling means and hence to said primary winding in use; and means, including a capacitor electrically connected to said coupling means to be chargeable by spark-producing pulses from said primary coil in use, for applying and maintaining a voltage at the control electrode of the transistor of a magnitude and duration to maintain a switched condition of said transistor from its normal condition prior to operation or energization of said circuit, as long as the frequency of said spark-producing voltage pulses is above a predetermined level, not less than one every two seconds, thereby controlling the condition of said silicon controlled rectifier independently of an electrical connection across said coupling means for connecting said circuit to a mechanically actuatable switch.
 14. In combination, an internal combustion engine with a magneto ignition system that has a primary winding that can be shorted to prevent ignition, a manually shiftable drive transmission connected to said engine, a position responsive safety switch connected to and operated by said transmission, and an ignition interlock for controlling the starting of the internal combustion engine, said interlock being dependent in operation solely upon magneto-generated energy and comprising: first circuit means including a first electronic control means connected across said winding and controllable through a control electrode between a low impedance or conductive state and a high impedance or nonconductive state in response to a voltage signal; second circuit means for controlling the application of a voltage signal produced in said primary winding to said control electrode, including said position-responsive switch and a second electronic control means for selectively permitting a voltage signal to be applied to said control electrode; and third circuit means, including a voltage sensitive memory, for applying a signal to said second electronic control means and affectiNg the conductivity thereof as long as a voltage pulse rate above a predetermined level is produced in said primary winding, thereby controlling the condition of said first electronic control means independently of said position-responsive switch when said voltage pulse rate is at said predetermined level.
 15. A magneto safety interlock for controlling the starting of an internal combustion engine that has a magneto primary winding that can be shorted to prevent ignition, said interlock comprising: means, including a circuit adapted to be connected electrically to a primary winding of a magneto, to selectively short the winding, said circuit including an electronic switch electrically operable through a control electrode between a condition exhibiting high electrical impedance and a condition exhibiting low electrical impedance; and a control circuit coupled to said switch and adapted to be connected electrically to said winding for selectively changing said switch from a condition exhibiting a high electrical impedance to a condition exhibiting a low electrical impedance, including a position-responsive switch that, when closed, prevents the condition of said electrically-operable switch from being so changed; and electrical circuit means coupled to said electrically operable switch to prevent the condition of said electrically operable switch from being changed in response to opening said position-responsive switch when electrical pulses are being generated by said winding, said circuit means including a capacitor and serving to selectively isolate the control electrode of said electrically operable switch from positive pulses from said magneto primary winding; the electrical operation of said circuits and said electrical circuit means being dependent solely upon magneto-generated energy.
 16. A safety interlock for controlling the starting of an internal combustion engine that has a magneto ignition system with a primary winding that can be shorted to prevent ignition; said interlock comprising: a first electronic control means electrically connected across a primary winding of an ignition system, normally non-conductive and rendered conductive by an electrical potential applied to a control electrode thereof; a mechanically actuated switch and a second electronic control means, both electrically coupled to the primary winding and to the first electronic control means to control the electrical potential at said control electrode, said switch having a first condition that prevents an electrical potential from being applied to said control electrode and a second condition that does not prevent an electrical potential from being applied to said control electrode, and said second electronic control means having a condition in which it does not prevent an electrical potential from being applied to said control electrode, and a condition responsive to running of the engine in which it prevents an electrical potential from being applied to said control electrode; and circuit means electrically connected to the primary winding and the second electronic control means, responsive to electrical pulses generated through said ignition system by the running of the engine, to apply an electrical potential to a control electrode of said second electronic control means to establish the condition of said second electronic control means in which it prevents an electrical input to the control electrode of said first electronic control means, whereby a change from said first condition of said switch subsequent to the starting of said engine will not render said first electronic control means conductive.
 17. A safety interlock as set forth in claim 16 wherein said second electronic control means is in series electrically with the control electrode of said first electronic control means.
 18. A safety interlock as set forth in claim 17 wherein the electrical impedance of said second electronic control means is normally low in the absence of a signal to its control electrode.
 19. A safety Interlock for controlling the starting of an internal combustion engine that has a magneto ignition system with a primary winding that can be shorted to prevent ignition; said interlock comprising: A first electronic control means electrically connected across a primary winding of an ignition system, normally non-conductive and rendered conductive by an electrical potential applied to a control electrode thereof; a mechanically actuated switch and a second electronic control means, both electrically coupled to the primary winding and to the first electronic control means to control the electrical potential at said control electrode, said switch having a closed condition that prevents an electrical potential from being applied to said control electrode and an open condition that does not prevent an electrical potential from being applied to said control electrode, and said second electronic control means having a condition in which it does not prevent an electrical potential from being applied to said control electrode, and a condition responsive to running of the engine in which it prevents an electrical potential from being applied to said control electrode; and circuit means electrically connected to the primary winding and the second electronic control means, responsive to electrical pulses generated through said ignition system by the running of the engine, to apply an electrical potential to a control electrode of said second electronic control means to establish the condition of said second electronic control means in which it prevents an electrical input to the control electrode of said first electronic control means, whereby an opening of said switch subsequent to the starting of said engine will not render said first electronic control means conductive.
 20. A safety interlock as set forth in claim 19 wherein said circuit means includes capacitance means responsive to periodic electrical input pulses for applying an electrical signal to the control electrode of said second electronic control means for sufficient time to maintain a changed condition of said second electronic control means for the interval between said periodic pulses while said engine is running.
 21. A safety interlock for controlling the starting of an internal combustion engine that has a magneto ignition system with a primary winding that can be shorted to prevent ignition; said interlock comprising: a first electronic control means electrically connected across a primary winding of an ignition system, normally non-conductive and rendered conductive by an electrical potential applied to a control electrode thereof; a mechanically actuated switch and a second electronic control means, both electrically coupled to the primary winding and to the first electronic control means to control the electrical potential at said control electrode, said switch having a first condition that prevents an electrical potential from being applied to said control electrode and a second condition that does not prevent an electrical potential from being applied to said control electrode, and said second electronic control means having a condition in which it does not prevent an electrical potential from being applied to said control electrode, and a condition responsive to running of the engine in which it prevents an electrical potential from being applied to said control electrode; and circuit means electrically connected to the second electronic control means, responsive to the running of the engine, to apply an electrical potential to a control electrode of said second electronic control means to establish the condition of said second electronic control means in which it prevents an electrical input to the control electrode of said first electronic control means, whereby a change from said first condition of said switch subsequent to the starting of said engine will not render said first electronic control means conductive.
 22. A safety interlock for controlling the starting of an internal combustion engine that Has a magneto ignition system with a primary winding that can be shorted to prevent ignition; said interlock comprising: a circuit to be electrically connected across a primary winding of an ignition system and connected to contacts of a position responsive switch that in a first condition permits a potential to be maintained across the primary winding so that the engine can be started and in a second condition prior to the starting of the engine prevents such a potential from being maintained; said circuit including: a first electronic control means, with first, second and control electrodes, that is nonconductive until a gating signal is applied to the control electrode, and that when connected across the primary winding of an ignition system and when conductive, will short the winding to prevent the ignition from firing; a second electronic control means with first, second and control electrodes, and a conductive and nonconductive condition dependent upon the presence or absence of an electrical signal at the control electrode, which signal originates from said primary winding, said second electronic control means in one said condition permitting a gating signal to be effectively applied to the control electrode of said first electronic control means, said gating signal originating from the primary winding when the position responsive switch is in said second condition, and said second electronic control means preventing a gating signal from being effectively applied to the control electrode of said first electronic control means when said second electronic control means is in the other of said conditions, regardless of the condition of said position responsive switch; and means, coupled between said primary winding and the control electrode of said second electronic control means, responsive to electrical pulses from said primary winding, to apply an electrical signal to said last-mentioned control electrode to change the condition of said second electronic control means after the engine has started and to maintain a signal as long as said electrical pulses occur at a predetermined minimum frequency, thereby preventing a gating signal from being applied at the control electrode of said first electronic control means when said position responsive switch is changed to said second condition.
 23. A magneto safety interlock for an engine ignition system, including a silicon controlled rectifier for connection across a primary magneto winding of an engine ignition system, said silicon controlled rectifier being normally nonconductive and which when conductive serves to electrically short said winding; first electrical circuit means coupled to a control electrode or gate of said silicon controlled rectifier and for connection to a position responsive switch and also across said primary winding for changing the conductive condition of said silicon controlled rectifier in response to the conductive condition of said switch; and second electrical circuit means coupled to said control electrode or gate, responsive to running of said engine, for preventing the conductive condition of said silicon controlled rectifier from being changed in response to the conductive condition of said switch after said engine is running.
 24. A magneto safety interlock for selectively shorting electrical pulses only of a polarity used for ignition and produced in a primary winding of a magneto in an ignition system of an internal combustion engine, comprising: a silicon controlled rectifier for connection across a primary magneto winding of an engine ignition system, said silicon controlled rectifier being normally nonconductive and having a gate electrode that renders said silicon controlled rectifier conductive in response to an electrical signal; and electrical circuit means coupled to said silicon controlled rectifier, including (a) rectifier means for permitting the passage through said circuit means of only electrical pulses of the polarity used for firing said engine, (b) means for coupling said Gate electrode to said winding through a position-responsive switch that permits or prevents the change of the conductive condition of said silicon controlled rectifier in response to an electrical pulse prior to the running of the engine, depending upon the condition of said switch, and (c) capacitor means responsive to electrical pulses from said primary winding after the engine is running for preventing said silicon controlled rectifier from being rendered conductive by a condition of said position responsive switch.
 25. A safety interlock for controlling the starting of an internal combustion engine that has a magneto ignition system with a primary winding that can be shorted to prevent ignition; said interlock comprising: an electronic control means electrically connected across a primary winding of an ignition system, normally non-conductive and rendered conductive by an electrical potential applied to a control electrode thereof; a control circuit including a mechanically actuated switch electrically coupled to the primary winding and to the first electronic control means to control the electrical potential at said control electrode, said switch having a closed condition that prevents an electrical potential from being applied to said control electrode and an open condition that does not prevent an electrical potential from being applied to said control electrode; and circuit means responsive to running of the engine to prevent an electrical input from being applied to the control electrode of said electronic control means, whereby an opening of said switch subsequent to the starting of said engine will not render said electronic control means conductive.
 26. A safety interlock as set forth in claim 25 wherein said circuit means includes a capacitor coupled between said primary winding and said control electrode, charged by pulses generated through said primary winding, for preventing an electrical input to the control electrode of said electronic control means when said engine is running.
 27. An electrical circuit for selectively shorting only negative electrical pulses produced in a primary winding of a magneto in an ignition system of an internal combustion engine, comprising: a first electrical circuit connected to a primary winding of a magneto in an ignition system and including first electronic control means having a first, second and control electrode and operative when an electrical signal is applied to the control electrode to conduct in only one direction; a second electrical circuit connected to said winding and coupled to said control electrode for selectively controlling the conductivity of said first electronic control means; rectifier means for permitting only negative voltage pulses from said winding to be applied to said circuits; said second circuit including a position-responsive switch operable to selectively prevent or allow said first electronic control means from being or to be rendered conductive and a second electronic control means operable to selectively prevent or allow said first electronic control means from being or to be rendered conductive when said position-responsive switch is in a position to allow the first electronic control means to be rendered conductive, said second electronic control means having a control electrode and being operable between a condition exhibiting high electrical impedance and a condition exhibiting low electrical impedance in response to an electronic signal applied to its control electrode; and a third circuit means for applying an electrical signal to the control electrode of said second electronic control means, initially only when said position-responsive switch is operated to prevent said first electronic control means from being rendered conductive, to change the condition of said second electronic control means from that condition it exhibits in the absence of a signal applied to its control electrode to prevent said first electronic control means from being rendered conductive notwithstanding the subsequEnt condition of said position responsive switch. 